Conventionally, as a lifter mechanism having a lifting mechanism part which lifts or lowers a seat, for example, there is a lifter mechanism disclosed in Patent Document 1. This lifter mechanism disclosed in Patent Document 1 has upper rails of a left slider and a right slider, a left link and a right link pivotally coupled to side frames of a cushion frame unit disposed on an upper side thereof and also coupled to each other via a shaft, and one lifting mechanism part which lifts or lowers the side frame on one side (door side) with respect to the slider on the one side (door side). The lifting mechanism part has a sector gear rotatably attached to the vicinity of a forward-backward center inside the side frame on the one side, a connector coupling a rear link on the one side and the sector gear, and an output gear cooperating with an operating member meshing with the sector gear. By rotating the output gear via the operating member, the link on the one side is swung via the sector gear and the connector meshing therewith, thereby lifting or lowering the side frame on the one side with respect to the slider on one side. Then, accompanying this lifting or lowering operation of the side frame on the one side, as described above, the side frame on the other side coupled via the shaft is lifted or lowered.